In the field of rock mechanics, it is often necessary to measure the stress characteristics of a medium in order to accurately assess the stability of certain rock formations. In particular, it is important to measure the state of stress in salt or other soft creeping rock types which may be used in waste disposal and storage. This information is extremely important with regard to radioactive waste, and nuclear waste storage designers will require accurate in situ readings of stress in a particular soft rock mass in order to plan for safe and adequate storage facilities.
A wide variety of methods have been devised over the years for the purpose of measuring the change in the state of stress of a rock mass which involve placement of an instrument inside of a borehole. All of these previous attempts at accurate measurement of soft rock types were inadequate because the instruments are mostly too stiff and often deform the rock continually during use. The open borehole tends to close under the state of stress by creep, plastic flow or by changes in the state of stress. When a stiff instrument is placed in the borehole, it restricts the hole closure. As a result, the instrument establishes its own stress field around the hole that completely destroys the hole behavior produced by the stress field "far away". The instrument can not then define the rock stress in an adequate way because it is responding to the secondary field it has produced itself.
Some of these rigid prior art borehole devices are known in the patent art. For instance, U.S. Pat. No. 4,574,485 (Kreutz et al.) discloses a borehole measuring device comprised of two longitudinal parts which are kept in contact with the borehole wall by means of a spring. U.S. Pat. No. 4,596,151 (Allwes) discloses a borehole pressure sensor which employs piston assemblies pressed against the borehole walls to detect stress changes, and uses hydraulic fluid to maintain the pistons in contact with the wall. U.S. Pat. No. 4,389,896 (Babcock) discloses a method for gauging boreholes which involves placement of two inclusions inside a borehole, each inclusion having a strain rosette sensor to measure physical properties. Finally, U.S. Pat. No. 3,796,091 (Serata) discloses a borehole stress gauge that consists of two axially movable tubular members fitted with diameter-measuring transducers. Other methods and devices for measuring stress properties in rocks using boreholes and other means are disclosed in Erer et al., Mining Sci. Tech. 2:191 (1985).
In addition to creating artificial stresses, some of the above devices are dangerous to use because they employ electric signals which could ignite explosive gases often found in rock formations. At present, there does not exist a device that can safely and accurately measure changing diameters of boreholes in soft creeping rock types either because of the interference on the rock caused by the device itself or the potentially explosive electric signalling means employed in these prior art devices. What is desired, therefore, is a safe, convenient and effective device which can accurately gauge the change in diameter of a borehole without applying any appreciable force on the wall of the borehole, and without the need for potentially dangerous electric signalling.